At the present time, claw-pole generators are customarily used to produce the electrical power required in a motor vehicle. These claw-pole generators are three-phase generators whose output current is rectified using a diode bridge. The rectified current is then used to supply the electrical loads of the vehicle and to charge the battery.
Such a three-phase generator includes a field coil through which field current flows. The field current is regulated with the aid of a voltage regulator so that the output voltage of the generator is approximately constant independently of the rotational speed of the generator. The level of voltage at which the regulator is set is customarily selected so that it is optimally suited for charging the battery. Depending on various conditions, the voltage is between approximately 12 and 14.5 V.
In vehicle electrical systems with a large number of loads, there is the problem that a 12-V system is not adequate for the supply of power. In particular for the supply of loads requiring a higher voltage than 12 V, there are conventional methods by which the generator is regulated at least temporarily to higher voltages such as 40 V, for example. This higher voltage is then made directly available to the load in question. The lower voltage required for the supply of the vehicle electrical system or for battery charging is derived from the higher voltage with the aid of a DC-DC converter. Since, in addition to power transistors, diodes and capacitors, conventional DC-DC converters also require inductive components, they are relatively expensive. This applies to transformer (potential-free) converters with a transformer and filter choke and to non-floating choke transformers with a storage choke. Such a generator control with a DC-DC converter is described in European Patent No. 0 325 520.